1. Field of the Invention
The present invention relates to an antenna device and a radio apparatus having a broadband characteristic, and in particular to an antenna device having multiple resonances and a radio apparatus including the antenna device.
2. Description of the Related Art
Known is a broadband antenna device as disclosed in Japanese Patent Publication of Unexamined Applications (Kokai), No. 2002-64324, particularly in FIG. 10. The antenna device of JP 2002-64324 has a ground plane 6 and a planar microstrip antenna 42 arranged parallel to the ground plane 6 , and is configured in such a way that an end of the microstrip antenna 42 is connected to an end of a monopole antenna 1.
The antenna device of JP 2002-64324 has a single resonance. The monopole antenna 1 is about a half as long as a wavelength of a resonant frequency. The planar microstrip antenna 42 is also about a half as long as the wavelength. The planar microstrip antenna 42 may increase its width and thus its electric volume so as to obtain a broad bandwidth.
Known is a planar multi-layered antenna of multiple resonances as disclosed in Japanese Patent Publication of Unexamined Applications (Kokai), No. 2005-94501, particularly in FIG. 5. The planar multi-layered antenna of JP 2005-94501 has a rectangular conductor pattern 43 and a U-shaped line conductor pattern 45. The rectangular conductor pattern 43 is arranged on a same plane as a ground board conductor 49.
The planar multi-layered antenna of JP 2005-94501 has multiple resonances, a first resonant frequency f1 of a current resonance on the U-shaped line conductor pattern 45 as a whole, and a second resonant frequency f2 of a resonance along an inner side of the U-shaped portion of the line conductor, where f1<f2.
Although having obtained a broadband characteristic, the above antenna device of JP 2002-64324 does not have a multi-resonance characteristic. Although the planar multi-layered antenna of JP 2005-94501 has resonant frequencies determined by the lengths of the whole U-shaped line conductor pattern 45 and of the inner side of the U-shaped portion of the line conductor, how to broaden the frequency bands is not very specifically disclosed.